The invention relates to a synthetic, refractory material for refractory products.
In the text which follows, the term resistor denotes a material which is the main component of a refractory product. In the most general situation, this resistor may be a metal-oxide, mineral, refractory material, such as MgO, Al2O3, doloma or the like.
In the text which follows, the term elasticizers denotes mineral, elasticizing materials which have a relatively high refractory quality and, on account of a thermal expansion which differs from that of the resistor and on account of the resultant microstructural defects, such as for example microcracks, in particular along the grain boundaries, and further effects lead to an increase in the thermal shock resistance of a mixture of resistor and elasticizer, compared to the pure resistor.
Refractory products, in particular basic refractory products based on magnesia, doloma, chromite and/or spinel (MgAl2O4) are used in all high-temperature processes with basic slag attack, such as for example in the production of cement, lime, dolomite, iron and steel and for the production of nonferrous metals and in the glass industry as lining material for furnaces, vessels and treatment units. However, if they have a high refractory quality and good chemical resistance, these materials or shaped bodies are highly brittle, i.e. have a high modulus of elasticity, resulting in adverse effects on the service life with regard to thermal expansion, stresses, mechanical loads and thermal shock resistance (TSR).
Furthermore, it is known for refractory shaped bodies also to be produced on the basis of Al2O3, in which case the raw material used is in particular bauxite, tabular alumina or fused corundum. Principal application areas for bricks of this type are electric furnace covers and ladles used in the steelmaking industry and cement kilns and the furnaces used in the glass industry.
It is known to reduce the high thermal expansion stresses of basic refractory products or shaped bodies by laying the refractory bricks with mortar joints, metallic inserts, such as metal sheets, perforated metal sheets or meshes, arranged between them.
Furthermore, numerous measures have been taken in the past to improve the thermal shock resistance, in particular even of basic refractory materials. It is known from Harders/Kienow, Feuerfestkunde, Herstellung, Eigenschaften und Verwendung feuerfester Baustoffe [Refractory technology, production, properties and use of refractory construction materials], Springer Verlag 1960, Chapter 5.5, pages 754 and 755, to considerably improve the thermal shock resistance by adding chrome ore (chrome magnesia brick) and by means of what is known as a miscibility gap, i.e. minimizing the mean grain size fraction (0.2 to 0.6 mm). However, a major drawback of the miscibility gap is, on the one hand, that its effect is only sufficiently high in combination with a TSR component, such as for example magnesia in chrome magnesia bricks or chrome ore in magnesia chrome bricks, if, on the other hand, when using the miscibility gap it is also impossible to achieve an optimum grain packing density as is desired in order to achieve a high resistance to infiltration with respect to slags. Furthermore, with regard to the addition of chrome ore (e.g. Harders/Kienow, page 754), the quantity of chrome ore and the optimum grain size fraction of the chrome ore have been defined. To achieve a sufficient TSR, quantities of chrome ore of between 15 and 30% by weight have been recognized to be suitable. The elasticizing action of the chrome ore in shaped bodies based on magnesia was hitherto unequalled. However, decisive drawbacks of the use of chrome ore as an elasticizer (TSR component) are that material fatigue occurs when the kiln or furnace atmosphere changes, and that the chromium oxide, which is present in trivalent form in the chrome ore, is converted by oxidation under the action of alkalis into toxic hexavalent chromium oxide, with all the associated problems in terms of safety at work and disposal.
It is known from Austrian patent AT 158208 to add alumina powder, corundum and aluminum powder to magnesia bricks in order to improve the TSR, spinel (MgO—Al2O3) being formed in situ during brick firing. The spinel formed is concentrated in the matrix material, which surrounds the resistor grains, and is in some cases not fully reacted, so that in the event of such bricks being attacked by slags, the matrix, which is of crucial importance for the strength, is preferentially destroyed. Furthermore, the improvement in TSR which can be achieved is limited, since the proportion of Al2O3 required to achieve a decisive improvement would have to be well over 8% by weight. On account of the excessive growth of the bricks as a result of an increase in volume in the matrix, however, this is impossible, since otherwise the dimensional accuracy and mechanical strength become insufficient and the porosity becomes excessive.
It has been possible to considerably improve both the TSR and the chemicals resistance of magnesia bricks by adding pre-synthesized magnesium-aluminum spinel in the form of sintered or fused spinel, the quantities added usually being between 15 and 25% by weight.
Furthermore, DE 44 03 869 C1 has disclosed a refractory ceramic batch which, as carrier of the refractory quality, substantially contains sintered MgO, with a spinel of the hercynite type being used as elasticizer. However, its resistance to basic slags is inadequate.